Listen carefully to the first few phrases of a recorded or live version of your song. On the instrument of your choice, find the pitch around which the chords seem to revolve. This note will correspond to the key in which you write the music.
Listen through the recorded or live version of your song and use the pencil and piece of scrap paper to jot down the instruments you hear.
Working from the woodwinds down through the brass, strings, and percussion instruments, assign one staff (two for keyboard instruments) on your music staff paper for each instrument you hear. If you just want to preserve the basics of the song, you probably can get by with just a vocal staff, piano staves, and a percussion staff.
Write in the clef for each instrument you've assigned and write in the key signature to the left of the clefs on each staff.
Listen to the recorded or live version of your song a few measures at a time to outline the rhythm for each measure of each instrument. Repeat the process for the chord progression.
Listen to the recorded or live version of your song a few measures at a time to sketch the general direction of the melodic line for each staff. Then go back and write in the specific pitches.
Put in any other notations that are needed, such as the dynamic markings, tempo markings, articulations, title and composer.
Listen to the recorded or live version of your music again to double check your work. Fix mistakes as you go.